Config ca_cert is now a PEM stringConfig ca_cert is now a PEM string
Config ca_key is now a PEM string
Expose ca cert and key as objects

They are still given to the Config object as PEM-encoded strings.
Config provides an openssl digest
Sign certificate if request valid
Remove pipe command support

Now that I've proven that we can sign with the openssl library, we can
throw away the buggy pipe command support.